A method of blasting an open cast blast hole

ABSTRACT

The invention provides a method of Wasting an open cast blast hole the method including placing one or more explosive charge in the blast hole, stemming the blast hole with stemming material above said one or more explosive charge, placing a propellant powder cartridge into the stemming material so that there is a layer of stemming material between the propellant powder cartridge and the explosive charge which is closest to the open end of the blast hole, and initiating the propellant powder charge with an igniter, or a detonator, or a fuse head, or any electronic initiation system.

FIELD OF THE INVENTION

The invention relates to blasting in open cast mining.

BACKGROUND TO THE INVENTION

The inventor is aware that in current open cast mining practices, blast hales are drilled and explosives are placed at or near the base of the blast, a detonator is inserted in the explosive, stemming material is tamped up against the explosive with a shock tube protruding out therethrough, further layers of explosives with detonators and stemming material follow, with a final stemming material layer closing up the blast hole while the shock tubes from the various detonators in the hole are is connected to an initiation system.

There are some draw backs to this system such as stemming material and rock being ejected from the blast hole during as a result of the detonation of the various layers of explosive in the blast hole.

The inventor thus proposes the invention described herebelow to reduce or even solve the above problems.

SUMMARY OF THE INVENTION

The invention provides a method of blasting an open cast blast hole, the method including:

-   -   placing one or more explosive charge in the blast hole;     -   stemming the blast hole with stemming material above said one or         more explosive charge;     -   placing a propellant powder cartridge into the stemming material         so that there is a layer of stemming material between the         propellant powder cartridge and the explosive charge which is         closest to the open end of the blast hole; and         initiating the propellant powder charge with an igniter, or a         detonator, or a fuse head, or any electronic initiation system.

The igniter may be a Maxfire™ from Master Blaster.

The explosive in the blast hole may be initiated by a detonator having a delay of from 350 ms to 20 000 ms, and the propellant powder charge initiation system may include a delay time which is at least 25 ms shorter than that of the detonator in the same blast hole. The propellant powder charge initiation system delay may thus be from 25 ms to 19 975 ms.

In a blast hole, starting from the bottom end thereof, there may be alternating explosive layers and stemming material layers with the propellant powder cartridge being located below the stemming material layer at the blast hole opening.

The propellant powder cartridge may be used similarly to how a pocket charge or satellite charge of explosive is used.

A preferred initiation method for the propellant powder cartridge is an igniter having a delay time of 400 ms difference with that of the explosive charge in the same hole below the propellant powder cartridge.

Blasting using the method may reduce the amount of fly rock and stemming ejection when compared to blasting using explosives only in the blast hole.

Prior to making the invention, the inventor was of the view that placing the propellant powder cartridge as in this invention would not work until he had conducted trials and determined the above parameters.

The inventor believed that that a propellant powder charge placed as per the invention and timed to initiate before the explosive charge in the blast hole would simply loosen all the rock above the explosive causing the loose rocks to become fly rock when the explosive is detonated in the blast hole and making the fly rock problem worse than when explosive alone is used. To the inventor's surprise, after conducting tests with the parameters of the invention stated above it was found not to cause the expected problem.

Example of Implementation of the Invention

A trial was conducted using the method of the invention.

In the trial blast holes were drilled and the holes were loaded with a main explosives charge for which an out of hole initiation of AEL™ shock tube system was used and thereafter a propellant powder cartridge, designated an RBC cartridge, was loaded and an out of hole igniter initiation system was used to initiate the propellant powder.

Stemming material was tamped in the hole above the main explosives i.e. below te RBC cartridge and again above the RBC cartridge and the hole opening.

A description of the holes, the explosives, and the charging method appear below.

Holes:

89 mm

Hole depth between 7-8 meters

B×S—2 meters×2 meters

Number of holes 400

+−20 Rows of 20 holes per row.

Explosives Products Used:

Main explosives charge—Bulk Emulsion supplied by BME.

In-hole initiation—AEL shock tube system.

Out-of-hole initiation—AEL shock tube system.

RBC cartridge—None×60 mm×270 mm (500 gram)

RBC initiation—100 ms Maxfires LD Igniter.

Charging and Timing Methods:

The first 4 lines of the blast (20 holes) were charged using the RBC cartridge in the stemming.

A base charge of 5 meters of explosives was used.

Stemming of 1-1.5 meters was placed above the explosives column.

The RBC was placed in the hole.

A further 750-1.730 meters of stemming was placed above the RBC.

The in-hole detonator was a 500 ms.

The in-hole Maxfires Igniter was 100 ms

Objective:

To place a 60 mm×500 gram RBC cartridge in the stemming portion of an explosives filled hole to creating a “pocket or satellite” type charge which will break the normally uncharged portion of the hole. This portion of the hole is usually filled with some kind of stemming material and cannot break thereby creating big boulders (oversize) which has to be dealt with at a later stage costing the operator both time and money. This operation is commonly referred to as secondary blasting.

It was envisaged that the trial would prove that explosives and propellant powders can be used in the same hole.

The RBC was initiated by a delayed igniter which will be at least 300 ms less than the delayed detonator used to initiate the explosives in the explosives filled portion of the hole. In this instance the timing will be a 100 ms delay in the igniter and a 500 ms delay in the detonator.

The explosives in the bottom portion of the hole in this instance were bulk emulsion manufactured by BME but could also be cartridge or Anfo.

The propellant powder cartridge (RBC) will break the portion of the rock in which it is located (normally filled only with inert stemming material) known as the “cap”. The RBC cartridge will initiate before the base charge moving the uncharged portion basically blocking off the hole so minimum glass ejection from the initiation of the explosives part of the hole is experienced. This operation will save the operator time and money in reducing secondary blasting.

A further crucial aspect of this operation is that this type of blasting will reduce the amount of “fly rock” making it safer for all concerned.

Results:

Initial results (as per the photograph below) show that the first 4 rows×20 holes has NO stemming ejection.

Stemming ejection from the portion of the blast without the RBC's in the stemming column show significant venting of gasses from a number of holes.

As shown in the accompanying photograph marked FIG. 1, the fragmentation in the “cap” from the first 4 rows is superior to the rest of the blast where the RBC was not used (320 holes) due to the confinement of the gasses in the RBC portion of the blast (first 4 rows).

Summary:

From the first blast of its kind anywhere in the world as shown in the accompanying photograph (FIG. 1), it is clear to us that this unique method of blasting has achieved the initial objectives.

The cap seems to have remained in place, blocked the holes preventing gas venting—if an explosives pocket charge was used the cap would have moved significantly.

There was no stemming ejection in the portion of the blast where RBC's were used. There was significant stemming ejection in the rest or “usual” part of the blast. 

1. A method of blasting an open cast blast hole, the method including: placing one or more explosive charge in the blast hole; stemming the blast hole with stemming material above said one or more explosive charge; placing a propellant powder cartridge into the stemming material so that there is a layer of stemming material between the propellant powder cartridge and the explosive charge which is closest to the open end of the blast hole; and initiating the propellant powder charge with an igniter, or a detonator, or a fuse head, or any electronic initiation system.
 2. A method as claimed in claim 1, wherein the explosive in the blast hole is initiated by a detonator having a delay of from 350 ms to 20 000 ms, and the propellant powder charge initiation system includes a delay time which is at least 25 ms shorter than that of the detonator in the same blast hole.
 3. A method as claimed in claim 2, wherein the propellant powder charge initiation system delay is from 25 ms to 19 975 ms.
 4. A method as claimed in claim 1, wherein in a blast hole, starting from the bottom end thereof, there are alternating explosive layers and stemming material layers with the propellant powder cartridge being located below the stemming material layer at the blast hole opening. 5.-8. (canceled)
 9. A method as claimed in claim 2, wherein in a blast hole, starting from the bottom end thereof, there are alternating explosive layers and stemming material layers with the propellant powder cartridge being located below the stemming material layer at the blast hole opening.
 10. A method as claimed in claim 3, wherein in a blast hole, starting from the bottom end thereof, there are alternating explosive layers and stemming material layers with the propellant powder cartridge being located below the stemming material layer at the blast hole opening.
 11. An initiation method for a propellant powder cartridge in which an igniter having a delay time of 400 ms difference with that of an explosive charge is placed in a same hole below the propellant powder cartridge. 